I'm playing around with a technique at the moment where I am towing a GPS on a float with my A flag, and the GPS time has been synched to my camera time. I have some software (expensive professional mapping program I'm afraid) that will automatically link the GPS track points to the photographs which lets you then plot the photo's onto a map or google or whatever you want so you can see where they are located spatially on your site. I am using it surveying shallow water sites where we have a lot of sunken pottery, amphorae, stone anchors and so on.
But for what you want when you have wreck co-ordinates you can do it in reverse. Program your wreck co-ordinates into the GPS, use the GoTo function to navigate you on a surface swim overhead the wreck. Then you can descend to the wreck. Either fix the GPS to a float with your Dive Flag on it so it stays with your towed SMB, or put it into a waterproof container before you defend and clip it off or put it in a drysuit pocket whatever you have available. For shallow dives I would just leave it on the SMB, no problem with pressure water-proofness etc.
It will work quite well in shallow water with a towed SMB, I have used this technique to swim back to marked sites a couple of times and it was fine.
P.